The invention relates to a system function which provides display of heading and attitude on displays in an aircraft, for example a head-up display (HUD), in the event of failures in certain equipment for normal attitude display. The system function, which in English is called Attitude and Heading Reference System and is abbreviated AHRS with reference to its initials, supplements the aircraft""s normal display for heading and attitude. This display is intended to help the pilot to recover from difficult attitudes and then facilitate return to base/landing.
In order not to lose attitude and heading display in an aircraft in the event of failure of a normally-used inertial navigation system (INS) a redundant system is required. In good visibility a pilot can fly by using the horizon as an attitude reference, but with great uncertainty as to the heading. In bad weather, in cloud and at night when the horizon is not visible, the pilot can easily become disoriented and thereby place the aircraft and him/herself in hazardous situations.
AHRS systems calculate, independently of normal systems, attitude angles (pitch and roll) and heading. Such a system continuously displays the position to the pilot on a display in the cockpit. The need for a redundant system for attitude may be so great that an aircraft is not permitted to fly without one.
Redundant systems in the form of an AHRS unit are available today. Such a unit contains among other things gyros which measure aircraft angle changes in pitch, roll and yaw. It also contains accelerometers and magnetic sensor. The accelerometers are used to establish a horizontal plane. The magnetic sensors are used to obtain a magnetic north end. This type of AHRS system in the form of hardware is costly and involves the installation of heavy, bulky equipment on the aircraft. To overcome this there is proposed in this description a synthetic AHRS which uses sensors existing in the aircraft, which are not normally intended for AHRS calculation and which therefore partly have significantly lower performance, instead of sensors of the type included in an AHRS unit.
The angles are calculated with the aid of existing sensors in the aircraft. The aim is to use existing angular rate gyro signals and support these with calculations based on other available primary data in the aircraft. Angular rate gyros are normally used in control systems and generally have substantially greater drift than gyros for navigation.
According to one aspect of the invention, a method is provided for synthetically calculating redundant attitude and redundant heading by means of data existing in an aircraft as specified in the claims.
Different forms of embodiment have been developed. In one embodiment the heading of the aircrafts available and in another embodiment the heading is calculated on the basis of a magnetic heading sensor. When the heading is available the calculations can be substantially reduced.
When the heading is available (redundant heading) attitude is calculated by weighting together the signals from the angular rate gyros in the flight control system of the aircraft, information from air data (altitude, speed, angle of attack) and information about heading (redundant heading).
When the heading is not available, attitude and heading are calculated according to one embodiment with the aid of Kalman filters by weighting together the signals from the angular rate gyros in the aircraft""s control system, information from air data (altitude, speed, angle of attack and sideslip angle) as well as information from an existing magnetic heading detector in the aircraft.
One advantage of a synthetic AHRS according to the aspect of the invention is that it works out substantially cheaper than conventional AHRS system based on their own sensors if existing sensors in the aircraft can be used. This also saves space and weight in the aircraft.